Bis-carbonic acid monoaryl esters which are derived fron aliphatic polyester-polyols can be prepared, for example, by reacting polyester-diols with diarylcarbonates in the presence of special catalysts according to German Patent Application Ser. No. P 2,651,639.2 which corresponds to U.S. Patent Application Ser. No. 845,174, filed Oct. 25, 1977, now U.S. Pat. No. 4,191,705. Products of this type are valuable starting materials for various block copolymers and are, therefore, readily transesterified, for example with bis-phenols. The compounds thus obtained can then be used for preparing, for example, polycarbonate/polyester block copolymers, as described in German Published Patent Specification No. 2,651,639.